1. Field of the Invention
This invention generally relates to a polymeric material useful for an implantable device, such as a stent.
2. Description of the Background
Although stents work well mechanically, the chronic issues of restenosis and, to a lesser extent, stent thrombosis remain. Pharmacological therapy in the form of a drug delivery stent appears to be a feasible means to tackle these issues. Polymeric coatings placed onto the stent serve to act both as the drug reservoir and to control the release of the drug. One of the commercially available polymer coated products is stents manufactured by Boston Scientific. For example, U.S. Pat. Nos. 5,869,127; 6,099,563; 6,179,817; and 6,197,051, assigned to Boston Scientific Corporation, describe various compositions for coating medical devices. These compositions provide to stents described therein an enhanced biocompatibility and may optionally include a bioactive agent. U.S. Pat. No. 6,231,590 to Scimed Life Systems, Inc., describes a coating composition, which includes a bioactive agent, a collagenous material, or a collagenous coating optionally containing or coated with other bioactive agents.
The nature of the coating polymers plays an important role in defining the surface properties of a coating. For example, a very low Tg, amorphous coating material can have unacceptable rheological behavior upon mechanical perturbation such as crimping, balloon expansion, etc. On the other hand, a high Tg or highly crystalline coating material introduces brittle fractures in the high strain areas of the stent pattern. For example, methacrylate or methacrylate polymers have been used as coating materials partially because that they have an alternate tertiary backbone structure which imparts stability to the polymer as a result of the stability of the C—C bond. This stability renders the polymer relatively impervious to attack by free radicals. However, poly(methyl methacrylate) (PMMA), a commonly used methacrylate polymer, is a hard material (Tg=85° C. to 105° C.) with low elongation (about 4%). Another common methacrylate polymer, poly(butyl methacrylate) (PBMA), is relatively soft (Tg=25-35° C.), but also has low elongation. A coating formed of a PMMA/PBMA blend, or block or random copolymer thereof, would be softer than a coating formed of a pure PMMA, but would have better elongation than PMMA. In addition, both PMMA and PBMA are hydrophobic polymers. A coating formed of either polymer or a blend of the two polymers would have relatively low water uptake (about 0.4%) and would be relatively impermeable to certain drugs. A membrane formed by said polymer or polymer blend would be quite impermeable to poorly water soluble drugs such as paclitaxel, sirolimus and everolimus, etc. Such a coating would have limited application.
The present invention addresses such problems by providing a polymeric material for coating implantable devices. By balancing hydrophilic and hydrophobic components, a variety of drugs can be delivered in a controllable manner.